Re: [PATCH v23 08/22] richacl: Compute maximum file masks from an acl

[Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, 2016-06-30 at 15:46 +0200, Andreas Gruenbacher wrote:
> Compute upper bound owner, group, and other file masks with as few
> permissions as possible without denying any permissions that the NFSv4
> acl in a richacl grants.
> 
> This algorithm is used when a file inherits an acl at create time and
> when an acl is set via a mechanism that does not provide file masks
> (such as setting an acl via nfsd).  When user-space sets an acl via
> setxattr, the extended attribute already includes the file masks.
> 
> Setting an acl also sets the file mode permission bits: they are
> determined by the file masks; see richacl_masks_to_mode().
> 
> Signed-off-by: Andreas Gruenbacher <agruenba-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Reviewed-by: J. Bruce Fields <bfields-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
>  fs/richacl.c            | 157 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/richacl.h |   1 +
>  2 files changed, 158 insertions(+)
> 
> diff --git a/fs/richacl.c b/fs/richacl.c
> index d0a4135..056228f 100644
> --- a/fs/richacl.c
> +++ b/fs/richacl.c
> @@ -181,3 +181,160 @@ richacl_want_to_mask(unsigned int want)
>  	return mask;
>  }
>  EXPORT_SYMBOL_GPL(richacl_want_to_mask);
> +
> +/*
> + * Note: functions like richacl_allowed_to_who(), richacl_group_class_allowed(),
> + * and richacl_compute_max_masks() iterate through the entire acl in reverse
> + * order as an optimization.
> + *
> + * In the standard algorithm, aces are considered in forward order.  When a
> + * process matches an ace, the permissions in the ace are either allowed or
> + * denied depending on the ace type.  Once a permission has been allowed or
> + * denied, it is no longer considered in further aces.
> + *
> + * By iterating through the acl in reverse order, we can compute the same
> + * result without having to keep track of which permissions have been allowed
> + * and denied already.
> + */
> 

Clever!


> +
> +/**
> + * richacl_allowed_to_who  -  permissions allowed to a specific who value
> + *
> + * Compute the maximum mask values allowed to a specific who value, taking
> + * everyone@ aces into account.
> + */
> +static unsigned int richacl_allowed_to_who(struct richacl *acl,
> +					   struct richace *who)
> +{
> +	struct richace *ace;
> +	unsigned int allowed = 0;
> +
> +	richacl_for_each_entry_reverse(ace, acl) {
> +		if (richace_is_inherit_only(ace))
> +			continue;
> +		if (richace_is_same_identifier(ace, who) ||
> +		    richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace))
> +				allowed |= ace->e_mask;
> +			else if (richace_is_deny(ace))
> +				allowed &= ~ace->e_mask;
> +		}
> +	}
> +	return allowed;
> +}
> +
> +/**
> + * richacl_group_class_allowed  -  maximum permissions of the group class
> + *
> + * Compute the maximum mask values allowed to a process in the group class
> + * (i.e., a process which is not the owner but is in the owning group or
> + * matches a user or group acl entry).  This includes permissions granted or
> + * denied by everyone@ aces.
> + *
> + * See richacl_compute_max_masks().
> + */
> +static unsigned int richacl_group_class_allowed(struct richacl *acl)
> +{
> +	struct richace *ace;
> +	unsigned int everyone_allowed = 0, group_class_allowed = 0;
> +	int had_group_ace = 0;
> +
> +	richacl_for_each_entry_reverse(ace, acl) {
> +		if (richace_is_inherit_only(ace) ||
> +		    richace_is_owner(ace))
> +			continue;
> +
> +		if (richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace))
> +				everyone_allowed |= ace->e_mask;
> +			else if (richace_is_deny(ace))
> +				everyone_allowed &= ~ace->e_mask;
> +		} else {
> +			group_class_allowed |=
> +				richacl_allowed_to_who(acl, ace);
> +
> +			if (richace_is_group(ace))
> +				had_group_ace = 1;
> +		}
> +	}
> +	/*
> +	 * If the acl doesn't contain any group@ aces, richacl_allowed_to_who()
> +	 * wasn't called for the owning group.  We could make that call now, but
> +	 * we already know the result (everyone_allowed).
> +	 */
> +	if (!had_group_ace)
> +		group_class_allowed |= everyone_allowed;
> +	return group_class_allowed;
> +}
> +
> +/**
> + * richacl_compute_max_masks  -  compute upper bound masks
> + *
> + * Computes upper bound owner, group, and other masks so that none of the
> + * permissions allowed by the acl are disabled.
> + *
> + * We don't make assumptions about who the owner is so that the owner can
> + * change with no effect on the file masks or file mode permission bits; this
> + * means that we must assume that all entries can match the owner.
> + */
> +void richacl_compute_max_masks(struct richacl *acl)
> +{
> +	unsigned int gmask = ~0;
> +	struct richace *ace;
> +
> +	/*
> +	 * @gmask contains all permissions which the group class is ever
> +	 * allowed.  We use it to avoid adding permissions to the group mask
> +	 * from everyone@ allow aces which the group class is always denied
> +	 * through other aces.  For example, the following acl would otherwise
> +	 * result in a group mask of rw:
> +	 *
> +	 *	group@:w::deny
> +	 *	everyone@:rw::allow
> +	 *
> +	 * Avoid computing @gmask for acls which do not include any group class
> +	 * deny aces: in such acls, the group class is never denied any
> +	 * permissions from everyone@ allow aces, and the group class cannot
> +	 * have fewer permissions than the other class.
> +	 */
> +
> +restart:
> +	acl->a_owner_mask = 0;
> +	acl->a_group_mask = 0;
> +	acl->a_other_mask = 0;
> +
> +	richacl_for_each_entry_reverse(ace, acl) {
> +		if (richace_is_inherit_only(ace))
> +			continue;
> +
> +		if (richace_is_owner(ace)) {
> +			if (richace_is_allow(ace))
> +				acl->a_owner_mask |= ace->e_mask;
> +			else if (richace_is_deny(ace))
> +				acl->a_owner_mask &= ~ace->e_mask;
> +		} else if (richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace)) {
> +				acl->a_owner_mask |= ace->e_mask;
> +				acl->a_group_mask |= ace->e_mask & gmask;
> +				acl->a_other_mask |= ace->e_mask;
> +			} else if (richace_is_deny(ace)) {
> +				acl->a_owner_mask &= ~ace->e_mask;
> +				acl->a_group_mask &= ~ace->e_mask;
> +				acl->a_other_mask &= ~ace->e_mask;
> +			}
> +		} else {
> +			if (richace_is_allow(ace)) {
> +				acl->a_owner_mask |= ace->e_mask & gmask;
> +				acl->a_group_mask |= ace->e_mask & gmask;
> +			} else if (richace_is_deny(ace) && gmask == ~0) {
> +				gmask = richacl_group_class_allowed(acl);
> +				if (likely(gmask != ~0))
> +					/* should always be true */
> +					goto restart;
> +			}
> +		}
> +	}
> +
> +	acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED);
> +}
> +EXPORT_SYMBOL_GPL(richacl_compute_max_masks);
> diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> index 9102ef0..3559b2c 100644
> --- a/include/linux/richacl.h
> +++ b/include/linux/richacl.h
> @@ -178,5 +178,6 @@ extern void richace_copy(struct richace *, const struct richace *);
>  extern int richacl_masks_to_mode(const struct richacl *);
>  extern unsigned int richacl_mode_to_mask(umode_t);
>  extern unsigned int richacl_want_to_mask(unsigned int);
> +extern void richacl_compute_max_masks(struct richacl *);
>  
>  #endif /* __RICHACL_H */

Reviewed-by: Jeff Layton <jlayton-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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